Intergranular creep-cavity formation in low-alloy bainitic steels

Abstract

The dominant features of microstructure and stress state in the nucleation of cavities during creep of bainitic 2.25Cr-1Mo steel at 565°C have been determined. The ratio of maximum principal stress σ₁ to effective stress σ has been varied over a range of σ₁ from 60 to 208 MN mM⁻². The cavity density was found to be a linear function of σ₁ with no correlation for σ Detailed analysis of particles residing on-prior austenite grain facets has revealed that, contrary to previous inferences, cavities nucleate predominantly at grain-boundary sulphides, while carbides remain coherent over the range of stress state and strain examined. Sulphides continually substitute chromium for manganese and remain chemically stable for times up to 10000 h. Fractographic evidence indicates this may improve cohesion and be a contributory factor in the cavitation resistance exhibited by the high-chromium ferritics. Principal stress-controlled nucleation is consistent with classical nucleation theory with stable cavities forming on sulphide nuclei above a critical radius r _c (≃2/σ₁. A major implication is that creep strength and cavity nucleation susceptibility are independently controlled by the matrix carbide and sulphide dispersion respectively. Creep ductility should therefore be affected by those factors influencing the density of incoherent boundary particles. In particular, cavitation control by microalloying with sulphur-stabilizing elements may be possible.